Airbag devices are known with gas volume reduced by supplying gas to a portion of an occupant seat airbag to cause the airbag to deploy (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2007-106376, JP-A No. 2007-161201 and JP-A No. 2007-145098). Airbag devices are also known that deploy so as to form an indented portion between rear ends of a left and right pair of bags for restraining the head of an occupant (see for example JP-A No. 2006-88856 and 2006-88998).